How do you evaluate API usability?

As a consumer of an API, how do you tell how usable the API is? Do you learn though bitter experience? Do you experiment with small development projects to see what the experience is like or do you dive in head first with a larger project? Do you even think about evaluating API usability before you start using the API?

Can you get a feeling for how usable an API is by reading blog posts, forum posts etc?